Field
The Radar energy Absorbing Low Drag Vortex Generator (RAD-LDVG) relates to a class of devices which produce vortices over external or internal aerodynamic and/or hydrodynamic surfaces.
Background Art
Vortex generators (VGs) come in a wide variety of shapes and sizes, but are used to increase maximum lift coefficients, and/or reattach flows which would separate, and/or smooth unstable flows over lifting surfaces, stabilizers, high lift devices or flight control mechanisms. Some of the first mentions of devices which were intentionally designed to produce aerodynamic vortices were described in the 1930's. U.S. Pat. No. 1,994,045 describes an aircraft with a plurality of ridges in streamwise, spanwise and crosswise directions on wings and aerodynamic lifting surfaces. In normal flight, these forms of ridges set up Görtler vortices which will tend to keep flows attached at the expense of increased overall drag. These same structures were conceived in the Junkers design bureau of Germany in the late 1920's and implemented on a commercial scale on the Ju-52 of 1932. Pletschacher (2002) describes the corrugated Duraluminum skin which added to the comparative structural rigidity of the aircraft with respect to wood and canvas aircraft of the day. Although those early vortex-generating skins performed well in terms of flow attachment, high drag was a hallmark of the design. Accordingly, in 1936, as other design bureaus in Germany experimented with wing skins, it was found that smooth skins had significantly lower drag levels and were therefore preferred for most aircraft flight phases. As the Bayerische Flugzeugwerke Messerschmitt began flight testing of the Bf-110 “Zerstörer,” they discovered many aerodynamics challenges as reported by Mankau and Petrick, (2001). To solve some of these issues on the V1 version of the Bf-110 aircraft, several rows of rigid vortex-generating structures were installed on the upper surfaces of the wings. This is often recognized as the first application of separate, independent, replaceable VGs on an aircraft of any class. Although quite effective in delaying flow separation, they possessed one characteristic which would prove fatal in combat in later years: They generated radar cross-section spikes. Because they formed “corner” or “retro” reflectors, they had the tendency to return radar energy to the direction of the transmitting radar. Accordingly, modern stealth aircraft never employ VGs because of the large radar return echoes.
Following this first appearance of VGs, they were reproduced, altered and reapplied analytically, conceptually and experimentally. U.S. Pat. No. 2,163,655 shows an early attempt in the US to manipulate the effects of wingtip vortices via the application of diagonal flow slots cut from the pressure surface to the suction surface of the airfoil. Although the feature could indeed manage wingtip vortex strength, it did little to generate streamwise subscale vortical structures which are so critical in maintenance of attached flow, as had been proven by the Bayerische Flugzeugwerke Messerschmitt in 1936.
As aircraft flight speeds continued to push into the transonic regime, shock-induced separation became pronounced and often lead to flight control surface and flight path departure. To mitigate these effects, rigid structures like those presented in U.S. Pat. No. 2,532,753 were used. As with the Bf-109 VGs 20 years earlier, these, too possessed retro-reflector corners which would spike radar cross-section (RCS). Still, the Bell X-1B was one of the first aircraft to use separate, rigid VGs of this configuration to mitigate adverse transonic flow effects just ahead of the ailerons as shown by Winchester (2005).
Although radio frequency (RF) antenna designs are typically divorced from aerodynamic considerations, U.S. Pat. No. 2,270,314 shows a design for a “Corner Reflector Antenna” which is nearly identical to a conventional vane vortex generator. Clearly U.S. Pat. No. 2,270,314 teaches a device which is made from a single sheet of conductive material bent at 90° and roughly rectangular in form. Most of the myriad of vortex generators cited herein have features like this. Because a corner reflector or “retroreflector” can return radio frequency waves to the source, it is often used in surface objects like buoys and airborne targets of various classes because it makes them easy to track via a greatly increased radar cross section. Of course, if such structures are placed on the surface of an aircraft which is a typical application taught in nearly all of the VG references herein, the RCS of the aircraft employing such VGs will in turn be increased.
The rigid geometric shapes described in U.S. Pat. No. 2,800,291 which possesses retroreflector corner shapes, were used in the Bell X-1B. In spite of retro-reflector right angles, this marked the first time that rigid vortex generators of the wedge-ramp configuration were described in the patent literature. U.S. Pat. No. 3,072,368 describes more application of the rigid wedge-ramp configuration including flow reattachment on aircraft boattail structures and in the region of transonic shock pods on lifting surfaces. As with all preceding VG configurations described in the patent literature, they possess right angles and are rigid, and therefore very adversely impact total aircraft RCS.
Although the rigid ramp structures of U.S. Pat. No. 3,072,368 would clearly be effective in generating streamwise vortical structures, their consistent presence would incrementally induce drag during cruise. Again, RCS would be spiked in certain aspects because of the right angle designs taught. U.S. Pat. No. 3,578,264 addresses the issues of high persistent drag by employing rigid VGs which are completely submerged within the outer mold line of the airfoil itself. Accordingly, when they are not needed, they would produce no drag increment. When called upon, air could be actively blown through the wing over the rigid VGs and accordingly, keep the flow attached. Of course, once deployed, these rigid VGs would significantly increase RCS. When stowed, the holes through which they are retracted would possess their own high frequency RCS increments.
U.S. Pat. No. 3,672,718 shows how rigid VGs can be employed at the aft cab of automobiles while U.S. Pat. No. 4,039,161 shows rigid pop-up vortex generators that are used to attach flows over deflecting flight control surfaces. U.S. Pat. No. 4,320,919 shows a rigid VG skirt mounted to the roof of a ground vehicle. Cavity-mounted rigid VGs are shown on a truck in U.S. Pat. No. 4,343,506. U.S. Pat. No. 4,455,045 shows rigid doublet versions of ramp vortex generators on ground vehicles and aerodynamic surfaces. This design clearly works well when it comes to flow attachment, but is challenged with respect to RCS increment when made of radar-reflective materials like almost any form of metal as it teaches retro-reflector features. Further, because it is curvilinear in the vertical projection, its corner-reflector properties have even greater scatter than the designs taught by the conventional vane vortex generators of the Bf-110. If the VG of U.S. Pat. No. 4,455,045 were made of dielectric material, then the problem of radar transparency would reveal all aircraft surface imperfections which lie beneath the VG, also leading to RCS spikes.
U.S. Pat. No. 4,655,419 teaches rigid VGs which are nearly identical to those used by Bayerische Flugzeugwerke Messerschmitt in nearly identical arrangements, but on aircraft that fly on the lower edge of the transonic flight regime with swept wings. Because all of the aforementioned patents teach configurations which have retro-reflector features, their associated RCS increments will be comparatively large.
The first of the rigid “V” or “Wishbone” VGs are described in U.S. Pat. No. 5,058,837. These VGs function in much the same way as the rigid ramp VGs as a pocket of low energy flow is pushed into the apex of the structures prior to spilling over the lip. When considering steady flow, the gross flowfield structure and resulting vortical structures are nearly identical to those generated by ramp VGs of the same overall geometric dimensions. Again, because the sides are essentially normal to the surface of the aerodynamic structure, they form retro-reflectors and adversely impact RCS.
Another pop-up rigid VG configuration is laid out in U.S. Pat. No. 5,209,438. Like U.S. Pat. No. 4,039,161, these rigid VGs lie within the outer mold line of the aerodynamic surface and are thrust out from the outer mold line when driven by a cam or mechanical device. Just as is the case with the technology taught in U.S. Pat. No. 4,039,161, the RCS increment would be considerable.
This theme continues with U.S. Pat. No. 5,253,828 where the rigid VGs are mounted to the leading edges of control surface extensions in a way that is almost identical to U.S. Pat. No. 4,039,161 except that the VGs protrude through slots rather than holes. As with earlier pop-up VG configurations, the VG designs taught by U.S. Pat. No. 5,253,828 would possess not only high RCS increments when deployed, but their actuating slots and violation of the lifting surface outer mold line would further drive up RCS.
U.S. Pat. No. 5,755,408 shows another variant of retractable, rigid flow disruption devices, but with a mount near the leading edge of the aerodynamic surface rather than at the leading edge of the flight control surface. Another rigid device which can deploy from around the leading edge of an aerodynamic surface for generating vortical flow structures is described in U.S. Pat. No. 6,105,904. Like U.S. Pat. No. 5,755,408 and all of the preceding pop-up vortex generators, the lifting surface out mold line (OML) is violated by the extension-retraction mechanism and/or the VG itself, bringing both structural and RCS challenges.
U.S. Pat. No. 6,427,948 shows the first of the active VGs which is trained to bend in response to command signals. By using bimetallic or shape-memory-alloy (SMA) sheets and/or foils, the conventional vane VG of U.S. Pat. No. 6,427,948 is shown to change in camber. Because the VG is placed approximately normal to the flow, the induced drag of the VG may be manipulated by varying the amount of current flowing through the actuation mechanism. However, it contains no way to alter its height above the surface, angle with respect to the surface and therefore wetted area. Accordingly, only one component of its apparent drag may be reduced. Because of its rigid mount, it is incapable of being shaped to the compound curves present on many aerodynamic surfaces without gross rework, reshaping, or adapter rails or other mechanisms. What is more is that because these forms of VGs must be made from conducting materials like SMA's or bimetallic strips to function and they are charged with electricity, their RCS increments will be extremely large. Further, because low observables are important, their thermal actuation makes them stand out in the infrared spectrum which is directly counter to proper low-observables aircraft design.
U.S. Pat. No. 7,878,457 skirts the problem of OML penetration by employing conventional, rigid vane vortex generators which can be laid flat at a skewed angle to the oncoming flow. By doing so, the structural integrity of the aerodynamic surface is preserved and installation procedures are simplified. However, the complexity and small form factor of a folding mechanism which must fight full flight airloads at an angle which is normal to the surface is challenging. Because the deployed angle shown is “greater than 0° and less than 180°” to the surface, a pocket of low energy air will be found at the base near the actuation mechanism. Accordingly, this form of rigid VG is susceptible to ice accumulation and atmospheric fouling in that area, which can, as a result, freeze it in either the “up” or “down” position. Because these VGs are taught on the leading edge surfaces, conductant creepage will be a significant issue as flight through rain can render leading-edge surface actuators fundamentally ineffective, even with good seals. Because the VGs taught in U.S. Pat. No. 7,878,457 are also electrically actuated, the RCS in several spectra will be increased dramatically via several mechanisms including creeping wave radiation and surface gapping.
US Pat. Application 2005/0230565A1 shows a complex system of introducing vortical flow structures over flight control surfaces by using rigid channels, grooves and slots in the trailing edge of a wing structure, just ahead of a high lift mechanism. The complicated nature of this rigid system indicates considerable expense in design, fabrication and installation and occupation of nontrivial amounts of chordwise airfoil volume which decreases fuel storage capacity and therefore aircraft total range.
US Pat. Application 2011/0315248A1 describes the first of the rigid VGs to specifically address the issue of radar cross section by specifically claiming it to be compatible with “stealth technologies.” The technology taught in this application addresses the issue of RCS return from retro-reflector configurations which is a problem for nearly all previous VG configurations. Although 2011/0315248A1 addresses these concerns by employing oblique reflection techniques, several problems are generated by doing so. The first, as with earlier VG designs taught in nearly all preceding art is that the devices are rigid. Accordingly, they will be challenging to install on curvilinear surfaces which are the norm for most aircraft. Because the VGs are designed to employ oblique reflection techniques, they are necessarily RF reflective. Given that they are also rigid, surface gapping against the aircraft skins will be present. These surface gaps, themselves will present RCS leakage problems. The second RCS issue is that although monostatic radar is challenged by oblique reflection techniques, bistatic and semi-active radar tracking schemes are not. Rather, they rely upon high oblique angle RCS spikes for both general aircraft localization and terminal guidance. Because of the scale necessarily taught by 2011/0315248A1, the wavelength and multiples of the wavelength will be highly compatible with X and K-band radars which are very frequently used in the semi-active radars of surface-to-air and air-to-air guided missiles. Accordingly, a feature which is intended to help mitigate RCS issues will actually exacerbate them when considering modern air combat weapon systems, designs and tactics. The RAD-LDVG skirts all of the aforementioned concerns.